1. Field of the Invention
This invention relates to a linear motor-driven conveyor means having a transporting pallet which runs on a transporting track as it is driven by a linear motor.
2. Description of the Prior Art
As conventional conveyor means using a linear motor, there are available a conveyor means using only a linear step motor and a conveyor means using only a linear induction motor.
FIG. 1 shows an example of the conveyor means using a linear step motor. This conveyor means comprises a transporting pallet 71, wheels 72, a guide roller 73, a secondary side 74 of a linear step motor and a slit plate 75 which intercepts a photo-sensor 83. Rails 81, a primary side 82 of the linear step motor and photo-sensor 83 are fixedly positioned. The photo-sensor 83 is arranged in four pieces so designed that the sensor pattern varies with each single step movement of the linear step motor. This sensor pattern forms one pitch per eight steps in the case of the 1-2 phase excitation method of a 4-phase VR type (variable reluctance type) and therefore eight different sensor patterns are available. More particularly, four elements (decision of excitation phase, speed of the transporting pallet, moving direction of the transporting pallet and the detection of the position of the transporting pallet) are detected by four sensors for control purposes.
FIG. 2 is an example of the time-speed characteristic at the acceleration of a conveyor means using a linear step motor. The conveyor means using a linear step motor can easily carry out controlled speed reduction and positioning, but the maximum speed at acceleration (when leaving a station) is only about 1 meter/second, even if the supply of exciting current is increased, as shown by FIG. 2, and difficulty has been found in accelerating the conveyor means. This is because of the fact that if the frequency of the exciting pulse becomes high, it becomes impossible to supply sufficient electric current to coils (affected by the inductance of coils).
FIG. 3 shows a conveyor means using a linear induction motor. This conveyor means comprises a transporting pallet 71, wheels 72, a guide roller 73, a secondary side 76 of the linear induction motor and a slit plate 75 for intercepting a photo-sensor 83. On the fixed side of this conveyor means, there are rails 81, a primary side 84 of the linear induction motor and a photo-sensor 83.
FIG. 4 shows the composition of a controlling device of a conventional conveyor means using a linear step motor. Numeral 31 designates an outside apparatus which can be a computer, a control device such as a robot arranged on a transporting line side, push-buttons to be operated manually or the like, and gives pallet destination instructions. Numeral 32 designates a linear step motor controller which, when destination instructions are input from the outside apparatus 31, decides the accelerating direction of a pallet and controls acceleration, deceleration and positioning of a pallet by synchronizing with inputs from sensors. Numeral 33 designates a linear step motor driver which provides current amplification of exciting patterns of the linear step motor controller 32 and supplies exciting current to each phase of a linear step motor 34 of the 4-phase VR type (variable reluctance type). Numeral 35 designates a sensor arranged on the ground side (station side) which detects the speed, position information and moving direction of a pallet.
FIG. 5 shows the composition of a controlling device of the conventional conveyor means using a linear induction motor. Numeral 31 designates an outside device, the same as that in FIG. 4. Numeral 42 designates a linear induction motor controller which controls acceleration, deceleration and positioning of a linear induction motor 44. Numeral 43 designates a speed controlling device which varies the speed of the linear induction motor 44. An inverter controlling device and a voltage controlling device are generally used for this speed controlling device. Numeral 45 designates a sensor which performs the same as the sensor 35 in FIG. 4. Numeral 46 designates a switching unit for "ON" and "OFF" switching of a positioning device 47, normally a fine positioner.
FIG. 6 shows an example of the time-speed characteristic upon acceleration of a conveyor means using a linear induction motor. As shown in FIG. 6, the maximum speed at acceleration can be raised by making the capacity of a motor larger, but control of positioning is difficult. Although the use of electromagnets, the use of two linear induction motors for opposite excitation, and the use of a mechanically operated fine positioner and other devices have been attempted, it is difficult to effect positioning quickly and at a high precision.